1. Field of the Invention
The present invention relates to tunable filters used in telecommunications applications such as Dense Wavelength Division Multiplexing (DWDM), and more particularly to piezoelectric tunable filters.
2. Description of the Prior Art
DWDM is an essential technology to meet the growing need for increased communications system capacity. Current DWDM technology requires at least one filter for one DWDM channel. Each channel includes one filter, one multi-fiber collimator and one single-fiber collimator. Therefore, the more channels there are, the more filters are required. Thus conventional DWDM systems are very expensive and bulky. In addition, mechanical and optical reliability is limited due to there being a great many three-port devices in the system.
A tunable filter can be provided to resolve the above-mentioned problems. The tunable filter can selectively add or drop particular wavelength channels from a multi-wavelength network. Tuning mechanisms used include thermo-optic, electro-optic, and piezoelectric mechanisms.
Arrayed Waveguide Grating (AWG) technology is based on a thermo-optic mechanism. However, AWG systems are not directly tunable, and have high loss and slow tuning speed.
A Mach-Zehnder Interferometer typically uses electro-optic actuation. However, manufacturing of such interferometer is very complicated because four reflecting surfaces thereof need to be parallel.
A piezoelectric tunable Fabry-Perot filter is described in J. Stone and L. W. Stulz, xe2x80x9cPigtailed High-Finesse Tunable Fibre Fabry-Perot Interferometers With Large, Medium and Small Free Spectral Rangesxe2x80x9d Elect. Lett., Vol. 23, pp. 781-783 (Jul. 16, 1987). This article describes three Fabry-Perot filter prototypes and their test results. All three designs use standard lead zirconate titanate (PZT) piezoelectric components for tuning. These filters yielded finesse values up to 200. Furthermore, an insertion loss as low as 1.5 dB is observed for lower finesse values. These mechanically tunable filters have excellent wavelength selectivity and low insertion loss. Since they are mechanically tuned, however, they can only attain tuning speeds in the order of milliseconds.
A tunable optical filter is disclosed in U.S. Pat. No. 5,062,684, the disclosure of which is incorporated herein by reference. Referring to FIG. 3, a tunable optical filter 20 comprises two spaced ferrules 24 that are aligned along a common axis 28 and symmetrically oppose each other. Each ferrule 24 has an inmost end face 34, and a wafer 30 attached by adhesive material 31 to the end face 34. The wafer 30 has an inmost end face 38. A mirror 40 comprising alternating layers of material, at least one of which is a dielectric material, is embedded between the end face 34 and the wafer 30. An optical fiber 29 is disposed in a passageway 26 that is defined through the ferrule 24 and the wafer 30 along the axis 28. The two ferrules 24 are mounted in a support 42. The support 42 allows the ferrules 24 to be movable with respect to each other in directions along the axis 28. The movement may be accomplished by use of a piezoelectric transducer system 44. By applying a voltage across the transducer system 44, a distance between the opposing ends faces 38 of the wafers 30 can be changed. The higher the applied voltage, the greater the distance between the end faces 38. However, when the distance is changed, it is difficult to maintain accurate alignment between the optical fibers 29. This often causes high insertion loss.
Hence, a fast tunable filter that overcomes the above-mentioned problems is desired.
Accordingly, it is an object of the present invention to provide a tunable filter which can tune a wavelength of a desired channel and which has the capability to manage multiple wavelengths for different channels.
In order to achieve the above object, a piezoelectric tunable filter of the present invention comprises a thin film waveguide, a ring made of piezoelectric material surrounding the thin film waveguide, and an actuator surrounding the ring. The actuator generates acoustic waves that apply an external radial force on the ring. Because the ring has piezoelectric properties, a periodic, high frequency piezoelectric signal can be generated by the action of the acoustic wave. The piezoelectric signal can be input to the thin film waveguide, and produce lattice vibration of the thin film waveguide. The lattice vibration can generate a fluctuating pattern of refractive index in the thin film waveguide due to the modulating effect of the piezoelectric signal. That is, a predetermined radial force on the ring produces a desired vibration of the thin films of the thin film waveguide. Such vibration of the thin films of the thin film waveguide produces a desired refractive index in each thin film. An optical thickness of each thin film of the thin film waveguide is changed accordingly. Thus, a desired optical wavelength equal to the combined optical thickness of two optically adjacent high refractive index thin films or two optically adjacent low refractive index thin films of the thin film waveguide can be selected to pass through the thin film waveguide.
Other objects, novel features and advantages of the present invention will be more readily understood from the following detailed description of a specific embodiment thereof when read in conjunction with the accompanying drawings, in which: